1. Technical Field
The present invention relates to digital communication systems, and more particularly relates to a system and method for modeling distortion as a function of a quantization parameter.
2. Related Art
With the advent of personal computing and the Internet, a huge demand has been created for the transmission of digital data, and in particular, digital video data. However, the ability to transmit video data over low capacity communication channels, such as telephone lines, remains an ongoing challenge.
To address this issue, systems are being developed in which coded representations of video signals are broken up into video elements or objects that can be independently manipulated. For example, MPEG-4 is a compression standard developed by the Moving Picture Experts Group (MPEG) that utilizes a set of video objects. Using this technique, different bit rates can be assigned to different visual objects. Thus, the more important data (e.g., facial features) can be encoded and transmitted at a higher bit rate, and therefore lower distortion, than the unimportant data (e.g., background features).
Because bandwidth is at a premium, one of the important challenges is to be able to efficiently select bit rates that will meet the distortion requirements for each visual object. Ideally, a bit rate should be selected no higher than is necessary to ensure that the distortion for the visual object does not exceed a selected threshold. Unfortunately, because of the number of parameters that can be introduced in such encoding schemes, predicting distortion levels for corresponding bit rates is a complex problem. Accordingly, the process of selecting bit rates for visual objects remains a challenge.
One solution was taught in the paper entitled xe2x80x9cRate Control and Bit Allocation for MPEG-4,xe2x80x9d by Ronda et al., IEEE Transactions on Circuits and Systems For Video Technology, Vol. 9, No. 8, December 1999, which is hereby incorporated by reference (hereinafter xe2x80x9cRondaxe2x80x9d). In Ronda, a model was taught in which distortion D was defined as:
D(q)=a1q2+a2q+a3+N(0, "sgr"2), 
Where q is a quantization parameter, N is a Gaussian distribution, and a1, a2, a3 are distortion model parameters. (In MPEG-4 systems, the bit rate is a function of q.) One of the problems associated with this model, however, is that it provides a Gaussian distribution having polynomial mean and constant variance. Thus, it assumes a constant variance Gaussian distribution regardless of the value of q, which is generally inaccurate, particularly in the case of a low bit rate.
Accordingly, a need exists for a system that can more accurately model distortion in an encoding system.
This invention overcomes the above-mentioned problems, as well as others, by providing a distortion model in which distortion D(q) is calculated as a random variable that has a general Gaussian distribution that is a function of the quantization parameter q.
In a first aspect, the invention provides a method for determining a quantization parameter q that meets a predetermined quality level, comprising the steps of: providing a distortion model D(q)=N(a1q2+a2q+a3, b1q2+b2q+b3), wherein N is a Gaussian distribution and a1, a2, a3, b1, b2 and b3 are distortion model parameters; selecting a target distortion level; and calculating the quantization parameter q such that an upper bound of the distortion model D(q) is less than or equal to the target distortion level.
In a second aspect, the invention provides an encoding system having quality level selection capabilities, comprising: a system for selecting a target distortion level; a distortion model, wherein the distortion model determines a distortion level as a function of a quantization parameter, and wherein the distortion model includes a Gaussian distribution having a variance that is a function of the quantization parameter; and a system for calculating the quantization parameter such that the distortion level does not exceed the target distortion level.
In a third aspect, the invention provides a video encoder that allows for the selection of a distortion level, comprising: a selection system for selecting a target distortion level; and a system for determining a quantization parameter that will ensure compliance with the selected target distortion level, wherein the system includes an algorithm for calculating distortion that utilizes a Gaussian distribution having a variance that is a function of the quantization parameter.
In a fourth aspect, the invention provides a program product, stored on a recordable medium, which when executed allows for the selection of a distortion level in an encoding operation, comprising: a selection system for selecting a target distortion level; and a system for determining a quantization parameter that will ensure compliance with the selected target distortion level, wherein the system includes an algorithm for calculating distortion that utilizes a Gaussian distribution having a variance that is a function of the quantization parameter.